For advanced information on the basic layout and basic functions of the TCU and ULF-SBX-H, please consult the SBT.

Functional description, see BMW center Technology:

TIS -> Document -> SI Technology -> Enter SBT Number

SBT number: 840703056 (TCU variant US)

SBT number: 840104118 (TCU variant EURO)

SBT number: 840303045 (ULF-SBX-H)

found this in the wds version 12.0

I have no idea. I cannot find another such box in my car and in the e90 there are not too many places that it could be located as it is quite big. Especioally in the M3 which as minimal trunk space. Does this information relate to the e90? This description seems to me like an interim solution, perhaps one that was in use during 07.
I don't think it necessarily has any relevance to the retrofit kit where the instructions clearly state that you scrap the existing box (if you have BT) and install a new one, whilst also adding in the new wires to the existing connector.